Chapter 6 – Protein: Amino Acids
Learning Objectives
After completing Chapter 6, the student will be able to:
1. Describe how the chemical structure of proteins differs from the
structures of carbohydrates and fats.
2. (List the 9 essential amino acids.)
3. Trace the digestion of protein and list the enzymes needed to
complete the process.
4. Explain the process used by the body to synthesize new
proteins.
5. List the 8 major functions of protein in the body.
6. Describe nitrogen balance and provide examples of positive
nitrogen balance, negative nitrogen balance, and equilibrium.
7. Describe deamination, where it occurs in the body, the products
produced, and the fate of these products.
8. Discuss the factors used to evaluate protein quality.
9. Describe the diseases that result from inadequate intake of
protein and protein-kcalories.
10. Discuss the health effects of over-consumption of protein.
11. Calculate the protein needed daily using the RDA for protein.
12. Discuss the health risks of protein and amino acid
supplements.
13. Define nutritional genomics and explain its potential uses in
health care.
I.
The Chemist’s View of Proteins
Proteins are made from 20 different amino acids, 9 of which
are essential. Each amino acid has an amino group, an acid
group, a hydrogen atom, and a side group. It is the side group
that makes each amino acid unique. The sequence of amino
acids in each protein determines its unique shape and function.
A. Amino Acids
1. Have unique side groups that result in differences in the
size, shape and electrical charge of an amino acid
2. Nonessential amino acids, also called dispensable amino
acids, are ones the body can create. Nonessential amino
acids include alanine, arginine, asparagine, aspartic acid,
cysteine, glutamic acid, glutamine, glycine, proline, serine,
and tyrosine.
3. Essential amino acids, also called indispensable amino
acids, must be supplied by the foods people consume.
Essential amino acids include histidine, isoleucine,
leucine, lysine, methionine, phenyalanine, threonine,
tryptophan, and valine.
4. Conditionally essential amino acids refer to amino acids
that are normally nonessential but essential under certain
conditions.
B. Proteins
1. Amino acid chains are linked by peptide bonds in
condensation reactions.
a. Dipeptides have two amino acids bonded together.
b. Tripeptides have three amino acids bonded together.
c. Polypeptides have more than two amino acids bonded
together.
2. Amino acid sequences are all different, which allows for a
wide variety of possible sequences.
3. Protein Shapes
a. Hydrophilic side groups are attracted to water.
b. Hydrophobic side groups repel water.
c. Coiled and twisted chains help to provide stability.
4.
Protein Functions
a. Some carry and store materials.
b. Some provide strength.
c. Some require minerals for activation (example:
hemoglobin and the mineral iron).
5. Protein denaturation is the uncoiling of protein that
changes its ability to function.
a. Proteins can be denatured by heat and acid.
b. After a certain point, denaturation cannot be reversed.
II.
Digestion and Absorption of Protein
Stomach acid and enzymes facilitate the digestion of protein. It
is first denatured, then broken down to polypeptides. The small
intestine continues to break down protein into smaller peptides
and amino acids so it can be absorbed.
A. Protein Digestion
1.
In the Stomach
a. Protein is denatured by hydrochloric acid.
b. Pepsinogen (a proenzyme) is converted into its active
form pepsin in the presence of hydrochloric acid.
c. Pepsin cleaves proteins into smaller polypeptides.
2.
In the Small Intestine
a. Proteases hydrolyze protein into short peptide chains
called oligopeptides, which contain four to nine amino
acids.
b. Peptidases split proteins into amino acids.
B. Protein Absorption
1. Used by intestinal cells for energy or synthesis of
necessary compounds
2. Transported to the liver
3. Taking enzyme supplements or consuming predigested
proteins is unnecessary
III. Proteins in the Body
Proteins are versatile and unique. The synthesis of protein is
determined by genetic information. Protein is constantly being
broken down and synthesized in the body. Researchers measure
nitrogen balance to study synthesis, degradation and excretion
of protein. Protein has many important functions in the body.
Protein can be used for energy if needed; its excesses are stored
as fat. The study of proteins is called proteomics.
A. Protein Synthesis
1. Synthesis is unique for each human being and is
determined by the amino acid sequence.
2. Delivering the instructions through messenger RNA
a. Carries a code to the nuclear membrane and attaches to
ribosomes
b. Presents a list to make a strand of protein
3. Transfer RNA lines up the amino acids and brings them to
the messenger
4. Sequencing errors can cause altered proteins to be made.
An example is sickle-cell anemia where an incorrect
amino acid sequence interferes with the cell’s ability to
carry oxygen.
5. Nutrients and Gene Expression - Cells regulate gene
expression to make the type of protein needed for that cell.
a. Epigenetics refers to a nutrient’s ability to activate or
silence genes without interfering with the genetic
sequence.
B. Roles of Proteins
1.
Building Materials for Growth and Maintenance
a. A matrix of collagen is filled with minerals to provide
strength to bones and teeth.
b. Replaces tissues including the skin, hair, nails, and GI
tract lining
2. Enzymes are proteins that facilitate anabolic (building
up) and catabolic (breaking down) chemical reactions.
3. Hormones regulate body processes and some hormones
are proteins. An example is insulin.
4. Regulators of Fluid Balance
a. Plasma proteins attract water
b. Maintain the volume of body fluids to prevent edema
which is excessive fluid
c. Maintain the composition of body fluids
5.
Acid-Base Regulators
a. Act as buffers by keeping solutions acidic or alkaline
b. Acids are compounds that release hydrogen ions in a
solution.
c. Bases are compounds that accept hydrogen ions in a
solution.
d. Acidosis is high levels of acid in the blood and body
fluids.
e. Alkalosis is high levels of alkalinity in the blood and
body fluids.
6. Transporters
a. Carry lipids, vitamins, minerals and oxygen in the body
b. Act as pumps in cell membranes, transferring
compounds from one side of the cell membrane to the
other
7.
Antibodies
a. Fight antigens, such as bacteria and viruses, that invade
the body
b. Provide immunity to fight an antigen more quickly the
second time exposure occurs
8.
Source of energy and glucose if needed
9.
Other Roles
a. Blood clotting by producing fibrin which forms a solid
clot
b. Vision by creating light-sensitive pigments in the retina
C. A Preview of Protein Metabolism
1. Protein Turnover and the Amino Acid Pool
a. Protein turnover is the continual making and breaking
down of protein.
b. Amino acid pool is the supply of amino acids that are
available.
1. Amino acids from food are called exogenous.
2. Amino acids from within the body are called
endogenous.
2. Nitrogen Balance
a. Zero nitrogen balance is nitrogen equilibrium, when
input equals output.
b. Positive nitrogen balance means nitrogen consumed is
greater than nitrogen excreted.
c. Negative nitrogen balance means nitrogen excreted is
greater than nitrogen consumed.
3. Using Amino Acids to Make Proteins or Nonessential
Amino Acids – Cells can assemble amino acids into the
protein needed.
4. Using Amino Acids to Make Other Compounds
a. Neurotransmitters are made from the amino acid
tyrosine.
b. Tyrosine can be made into the melanin pigment or
thyroxine.
c. Tryptophan makes niacin and serotonin.
5.
Using Amino Acids for Energy and Glucose
a. There is no readily available storage form of protein.
b. Breaks down tissue protein for energy if needed
6.
Deaminating Amino Acids
a. Nitrogen-containing amino groups are removed.
b. Ammonia is released into the bloodstream.
c. Ammonia is converted into urea by the liver.
d. Kidneys filter urea out of the blood.
7.
Using Amino Acids to Make Fat
a. Excess protein is deaminated and converted into fat.
b. Nitrogen is excreted.
IV. Protein in Foods
Eating foods of high-quality protein is the best assurance to get
all the essential amino acids. Complementary proteins can also
supply all the essential amino acids. A diet inadequate in any of
the essential amino acids limits protein synthesis. The quality of
protein is measured by its amino acid content, digestibility, and
ability to support growth.
A.
Protein Quality
1.
Digestibility
a. Depends on protein’s food source
1.
Animal proteins are 90-99% absorbed.
2.
Plant proteins are 70-90% absorbed.
3.
Soy and legumes are 90% absorbed.
b. Other foods consumed at the same time can change the
digestibility
2.
Amino Acid Composition
a. The liver can produce nonessential amino acids.
b. Cells must dismantle to produce essential amino acids if
they are not provided in the diet.
c. Limiting amino acids are those essential amino acids
that are supplied in less than the amount needed to
support protein synthesis.
3. Reference Protein is the standard by which other proteins
are measured. Based on their needs for growth and
development, preschool children are used to establish this
standard.
4. High-Quality Proteins
a. Contains all the essential amino acids
b. Animal foods contain all the essential amino acids.
c. Plant foods are diverse in content and tend to be
missing one or more essential amino acids.
5. Complementary Proteins
a. Combining plant foods that together contain all the
essential amino acids
b. Used by vegetarians
6. A Measure of Protein Quality - PDCAAS (protein
digestibility-corrected amino acid score)
a. Compares amino acid composition of a protein to
human amino acid requirements
b. Adjusts for digestibility
B. Protein Regulation for Food Labels
1. List protein quantity in grams
2. % Daily Values is not required but reflects quantity and
quality of protein using PDCAAS.
V.
Health Effects and Recommended Intakes of Protein
Protein deficiency and excesses can be harmful to health.
Protein deficiencies arise from protein-deficient diets and
energy-deficient diets. This is a worldwide malnutrition
problem, especially for young children. High-protein diets have
been implicated in several chronic diseases.
A. Protein-Energy Malnutrition (PEM) – also called proteinkcalorie malnutrition (PCM)
1.
Classifying PEM
a. Chronic PEM and acute PEM
b. Marasmus, kwashiorkor, or a combination of the two
2.
Marasmus
a. Infancy, 6 to 18 months of age
b. Severe deprivation or impaired absorption of protein,
energy, vitamins and minerals
c. Develops slowly
d. Severe weight loss and muscle wasting, including the
heart
e. < 60% weight-for-age
f. Anxiety and apathy
g. Good appetite is possible
h. Hair and skin problems
3.
Kwashiorkor
a. Older infants and young children, 18 months to 2 years
of age
b. Inadequate protein intake, infections
c. Rapid onset
d. Some muscle wasting, some fat retention
e. Growth is 60-80% weight-for-age
f. Edema and fatty liver
g. Apathy, misery, irritability and sadness
h. Loss of appetite
i. Hair and skin problems
4.
Marasmus-Kwashiorkor Mix
a. Both malnutrition and infections
b. Edema of kwashiorkor
c. Wasting of marasmus
5.
Infections
a. Lack of antibodies to fight infections
b. Fever
c. Fluid imbalances and dysentery
d. Anemia
e. Heart failure and possible death
6.
Rehabilitation
a. Nutrition intervention must be cautious, slowly
increasing protein.
b. Programs involving local people work better.
B. Health Effects of Protein
1.
Heart Disease
a. Foods high in animal protein also tend to be high in
saturated fat.
b. Homocysteine levels increase cardiac risks.
c. Arginine may protect against cardiac risks.
2.
Cancer
a. A high intake of animal protein is associated with some
cancers.
b. Is the problem high protein intake or high fat intake?
3.
Adult Bone Loss (Osteoporosis)
a. High protein intake associated with increased calcium
excretion.
b. Inadequate protein intake affects bone health also.
4.
Weight Control
a. High-protein foods are often high-fat foods.
b. Protein at each meal provides satiety.
c. Adequate protein, moderate fat and sufficient
carbohydrate better support weight loss.
5.
Kidney Disease
a. High protein intake increases the work of the kidneys.
b. Does not seem to cause kidney disease
C. Recommended Intakes of Protein
2.
Protein RDA
a. 0.8 g/kg/day
b. Assumptions
1.
People are healthy.
2.
Protein is mixed quality.
3.
The body will use protein efficiently.
3.
Adequate Energy
a. Must consider energy intake
b. Must consider total grams of protein
4. Protein in abundance is common in the U.S. and Canada.
D. Protein and Amino Acid Supplements
1. Many reasons for supplements
2. Protein Powders have not been found to improve athletic
performance.
a. Whey protein is a waste product of cheese
manufacturing.
b. Purified protein preparations increase the work of the
kidneys.
3. Amino Acid Supplements are not beneficial and can be
harmful.
a. Branched-chain amino acids provide little fuel and
can be toxic to the brain.
b. Lysine appears safe in certain doses.
c. Tryptophan has been used experimentally for sleep and
pain, but may result in a rare blood disorder.
Not to be tested
VI.
Highlight: Nutritional Genomics
In the future, genomics labs may be used to analyze an
individual’s genes to determine what diseases the individual
may be at risk for developing. Nutritional genomics involves
using a multidisciplinary approach to examine how nutrition
affects genes in the human genome.
A. A Genomics Primer
1. Human DNA contains 46 chromosomes made up of a
sequence of nucleotide bases.
2. Microarray technology is used to analyze gene
expression.
3. Nutrients are involved in activating or suppressing genes
without altering the gene itself.
4. Epigenetics is the study of how the environment affects
gene expression.
5. The benefits of activating or suppressing a particular gene
are dependent upon the gene’s role.
B. Genetic Variation and Disease
1. Small differences in individual genomes
2. May affect a disease’s ability to respond to dietary
modifications
3. Nutritional genomics would allow for personalization of
recommendations.
4.
Single-Gene Disorders
a. Mutations cause alterations in single genes.
b. Phenylketonuria is a single-gene disorder that can be
affected by nutritional intervention.
5.
Multigene Disorders
a. Multiple genes are responsible for the disease.
b. Heart disease is a multigene disorder that is also
influenced by environmental factors.
c. Genomic research may be helpful in guiding treatment
choices.
d. Variations called single nucleotide polymorphisms
(SNPs) may influence an individual’s ability to respond
to dietary therapy.
C. Clinical Concerns
1. An increased understanding of the human genome may
impact health care by:
a. Increasing knowledge of individual disease risks
b. Individualizing treatment
c. Individualizing medications
d. Increasing knowledge of nongenetic causes of disease
2. Some question the benefit of identifying individual genetic
markers.
3. Even if specific recommendation can be made based on
genes, some may choose not to follow recommendations.
D. Nutrition on the Net
1.
www.wadsworth.com/nutrition
www.cdc.gov/genomics
2.
Q & As for Discussion
1. How does the chemical structure of proteins differ from the
structures of carbohydrates and fats?
2. Describe the structure of amino acids, and explain how their
sequence in proteins affects the proteins’ shapes. What are
the essential amino acids?
3. Describe protein digestion and absorption.
4. Describe protein synthesis.
5. Describe some of the roles proteins play in the human body.
6. What are enzymes? What roles do they play in chemical
reactions? Describe the differences between enzymes and
hormones.
7. How does the body use amino acids? What is deamination?
Define nitrogen balance. What conditions are associated
with zero, positive, and negative balance?
8. What factors affect the quality of dietary protein? What is a
complete protein?
9. How can vegetarians meet their protein needs without
eating meat?
10. What are the health consequences of ingesting inadequate
protein and energy? Describe marasmus and kwashiorkor.
How can the two conditions be distinguished, and in what
ways do they overlap?
11. How might protein excess, or the type of protein eaten,
influence health?
12. What factors are considered in establishing
recommended protein intakes?
13. What are the benefits and risks of taking protein and
amino acid supplements?